Field of the Invention
The present invention relates to a plastic molding apparatus and method. More particularly, the present invention relates to a suspended multi-port flow controller and apparatus which dispenses and disperses molten plastic material in a preselected manner to facilitate complete and rapid filling of an open product mold.
Description of the Prior Art
Molded plastic components are generally formed by creating a mold having a hollow cavity of the desired configuration. The mold sections may be opened and the lower portion may be filled with molten plastic material in the form of a fillet, or the mold may closed and the molten plastic material injected into the mold. The molten material may be injected under pressure, such as in injection molding; pressed within a split mold, such as in compression molding; or drawn by vacuum into the mold, such as in vacuum forming, for final shaping and cooling into the finished component. Each of these techniques requires certain physical characteristics of the plastic delivery method, mold and molten fill material. Typically, the molten plastic material is provided by the use of an injection molding machine or plastic extruder, which converts solid raw plastic pellets to a web of molten material. The plastic pellets, which may be homogeneous or heterogeneous, are combined, as necessary, in various combinations and ratios to form the desired molding material. Depending on the molding material components and the characteristics of the delivery method or mold, the web of molten material may be maintained within a variety of temperature and viscosity parameters. It is important that these parameters be continuously monitored and maintained to ensure even flow of the material and comprehensive filling of the mold. Additionally, the characteristics and performance of the finished part are closely related to the flow rate of the molding material, the method of delivery of this material into the mold, plastic and mold temperatures, the cooling period in the mold and a variety of other processing parameters. In a traditional injection molding process molten material is injected under pressure into a closed mold. Because of the high pressures required, it is often too expensive or impractical to produce large, voluminous or very heavy parts using this method. In a traditional compression molding process a fillet or other plastic charge may be placed into an open tool in a molten or solid state and pressure and/or heat applied accordingly until the part is formed. Plastic pellets may also be placed directly into the compression molding tool in raw form, heated in the tool while under pressure, and formed under pressure into the desired part. The pellet method is relatively slow, in that material must be heated in the mold at the same time compression occurs and subsequently, the mold must be cooled from its heated state to allow forming of the part under pressure. The fillet method is slow as well, since the fillet must be generated by an extruder in advance of placement in the tool and delivery of the fillet into the mold is generally manual or minimally mechanized. Additionally, use of a single or even multiple fillets, which must be compressed across the face of the mold, can result in built in material stresses because of the distance within the mold that molten material must be shifted during its curing and compression.
One of the most significant shortcomings of these prior molding methods relates to these stresses. Once the plastic material begins to set or cure, it creates a base physical configuration formed by the molecular bonds within the material. The plastic material will therefore always have a tendency to return to the base configuration. Forming the component after the cure process has begun will therefore create internal stress within the component, as it is deformed away from the base configuration. This may lead to lack of structural integrity or subsequent warping, shrinkage or other deformation as the component seeks to return to the base configuration which may be exacerbated under certain environmental conditions. As such, it is important that material remain adequately molten and viscous during its flow and forming process.
Additionally, the area where two fillets may meet and combine or a single fillet overlaps during the compression and cooling process can be a structural weak point, as the discrete material may not combine or integrate fully, even under high pressure. Higher pressures are generally required for higher distances that the fillet must be dispersed or for higher volumes of material that must be moved within the mold to its final destination for part formation.
Polk, Jr., U.S. Pat. No. 6,719,551, issued Apr. 13, 2004, and Polk, Jr., et al., U.S. Pat. No. 6,869,558, issued Mar. 22, 2005, describe a thermoplastic molding process and apparatus which includes an adjustable die gate member for independently controlling flow of molten plastic material into portions of an open mold. In these references, an extruder provides a web of molten plastic material directly to a multi-segment gated die which controls the flow of the molten material over an open mold. The mold is on a movable trolley and moves, relative to the gated die in a longitudinal manner along a preset path, such as a conveyor or track. The extruder provides a continuous flow of molten material to the multi-segment gated die, which contains a plurality of independently controlled flow gates. Each gate may be raised or lowered in order to enhance or retard the flow of molten plastic material therethrough. The gates are arranged laterally across the flow path of the molten plastic web, which flow path corresponds to the width dimension of the mold. The gated die is positioned above the mold travel path, which is perpendicular to the lateral arrangement of the gates of the die. In this manner, the flow of the web of molten material is passed through the open gates of the die and downwardly to the mold moving below. As the mold is displaced longitudinally along its travel path, the web of molten material flows over it, similar to a waterfall, filling the mold in the width dimension through the flow of the gates and in the length dimension by the longitudinal displacement of the mold relative to the gates. Through the sequential, preselected opening and closing of the gates, a web profile may be created which corresponds to any upstanding or depressed features of the mold. In this manner, lateral sections of the molten web material flow in volumes corresponding to the localized capacity of the mold sections. For example, to the extent the mold has a significant depression in one section, the corresponding gates may be opened more fully while that section of the mold passes thereunder, or the speed of the molds travel longitudinally may be slowed. This allows relatively more molten material to flow into that particular section of the mold than while the mold is moving at a steady pace. This reduces the need for molten material to flow from other parts of the mold in order to fill the large, open space. This segmented filling enhances the ability of the mold to be filled quickly and evenly, with a reduced chance of incomplete or uneven filling. This improves the speed of the molding process, as well as the quality of the finished component from a structural standpoint.
The Polk process is particularly adapted for flow of molten web materials of higher viscosities. The particular arrangement of the gates in a lateral orientation experiences reduced effectiveness as the viscosity of the web of molten material is decreased, based upon the nature of the flow gates. Furthermore, material begins to cure immediately upon being deposited into the open tool and as a result of its exposure to ambient air, the plastic material deposited earlier in the deposit cycle will be at a different temperature, viscosity and/or state of cooling during its compression, thus reducing the controllability of the process and the quality of the final part. Moreover, the system as described requires the use of some type of conveyor or transport system to displace the mold during the molding process.
Polk discusses the direct coupling of an extruder to an adjustable die gate member. These embodiments require that material flow through the die gate apparatus and into the open mold, generally at the rate of extrusion. This limits the material being deposited to no faster than the rate of extrusion. This results in longer cycle times than necessary to compress the component, as well as a longer time for plastic to cool, begin setting and lose viscosity before being compressed.
There remains a need, therefore, for a process and apparatus for more comprehensive and variable control of the flow of molten plastic material through the delivery apparatus and over the mold, especially in the event that the mold has a large proportion of varying depths or other complexities and for a more uniform material dispersion process that does not rely upon the movement of an open mold longitudinally during the filling step. There also remains a need for a method and apparatus that allows for intra-compression-cycle accumulation and storing of to-be-deposited plastic material in a molten state, thus allowing for the more consistent and rapid deposit of material into the open mold. This minimizes the delay from the start of material deposit until the material fill has been completed and the mold is ready to be compressed.